Light source in the form of a sealed beam ARC lamp including three reflective surfaces

ABSTRACT

A light source suitable for use in a projection system is described. The light source is in a form of an arc lamp including an arc gap defined between an anode and a cathode. The arc gap is positioned at the focal point of a parabolic reflector which is truncated at its focal plane. A spherical reflector oppositely directed to the parabolic reflector and concentric with the focal point of the parabolic reflector is arranged so as to direct light from the arc which has not been intercepted by the parabolic reflector back into the arc gap.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to light sources. The invention has particularalthough not exclusive relevance to light sources for use in aprojection system.

One form of projection system includes one or more spatial lightmodulators, each modulator being controllable so as to modulate anincident light beam. Each spatial light modulator is controlled bysignals from an input video signal, and may be used to modulate light ofa different colour, the coloured spatially modulated light beams thenbeing combined to form a beam which is projected onto a projectionscreen.

2. Description of Related Art

Spatial light modulators may take various forms. One example is a liquidcrystal light modulator as for example described in EP 401912. Anotherexample is a tiltable mirror device as for example disclosed in U.S.Pat. No. 4,856,863. Such tiltable mirror devices comprise an array ofmirrored elements, each element being arranged to be electrostaticallydeflectable between an "on" position in which light is reflected fromthe element onto a projection screen, and an "off" position in which thelight is directed towards a beam dump dependent on address signalsapplied to the array. Thus a spatially modulated beam is producedcomprising "white" areas corresponding to light from the "on" elementsand "black" areas corresponding to light from the "off" elements. Inpractice greyscales are produced by temporal modulation.

In order to illuminate spatial light modulators, it is necessary to usea high intensity light source in order to provide a substantiallyuniform light beam. It is also necessary for the overall dimension ofthe projection system, and thus the light source to be relativelycompact.

ILC Technology Inc of California, USA manufacture a compact, highintensity light source which may be used for projection systemsincorporating a number of spatial light modulators. This light sourcecomprises a compact xenon arc lamp arranged to operate with an inputpower supply of one kilowatt to produce a 5 cm diameter output beam.Such a light source, however, suffers the disadvantage that much of thelight beam does not lie within the visible spectrum. Furthermore thereis a limit to the amount of input power which may be supplied to thedevice due to problems of overheating.

In our copending International Patent Application WO 93/26034, there isdescribed an arc lamp suitable for use as a high intensity light sourcein a projection system incorporating a number of spatial lightmodulators. The contents of WO 93/26034 are incorporated herein byreference. In the arc lamp described in WO 93/26034, a parabolicreflector is arranged to reflect the light produced by the arc into adirectional light beam. Secondary reflection means are arranged toredirect part of the reflected beam to compensate for regions of thebeam which are obscured by one of the electrodes of the arc lamp.Various heat sinks are provided within the light source so as todissipate heat generated by the electrodes which define the arc.

The arc lamp described in WO 93/26034 will now be briefly described withreference to FIG. 1 which is a schematic, partially sectioned side viewof the arc lamp described in WO 93/26034.

Referring to FIG. 1, the arc lamp comprises a cathode 101 and an anode103 in a xenon atmosphere enclosed in an enclosure defined by aparabolic reflector 105 and a light emitting sapphire window 107 formedin the shape of a lens. The cathode 101 is supported by thin metallicsupports 109, and is connected to a DC voltage supply (not shown). Theanode 103 is connected to a battery via a conductive path through amounting including a heat sink 113.

In use of the lamp, an arc is struck between the anode 103 and cathode101. As the arc is arranged to be at the focal point of the parabolicreflector 105, light from the arc will be reflected by the parabola toform a substantially parallel beam directed out of the enclosure throughthe sapphire window 107. The light source is provided with an outerconical reflector 115, which is arranged to deflect light at theperiphery of the beam towards the central portion of the enclosure to bereflected by an inner conical reflector 117 whose reflective surface isparallel to that of the outer reflector. The outer conical reflector 117directs the light out of the window 107, thus compensating the centralpart of the output beam which is obscured by the presence of the cathode101.

Heat dissipation from the lamp is improved by the presence of coolingfins 119 formed on the cathode 101. Cooling fins 121 are also formed onthe heat sink 113.

A magnet 123, which may be a permanent magnet or an electro-magnet, isarranged in the anode mounting 113 so as to provide an axial magneticfield in the direction between the anode 103 and cathode 101. Thismagnetic field acts as a focusing field, reducing the diameter of thearc and thus reducing the divergence of the output beam. Inserts 125 and127 of a soft magnetic material may be used to concentrate the magneticfield produced by the magnet 123.

It will be seen that this prior art light source, suffers thedisadvantage that the output beam has a characteristic divergence andefficiency which is related to the size of the arc and to the focallength of the parabolic reflector 105. Thus the divergence is related tothe size of the lamp and in order to reduce the divergence or increasethe efficiency of the output beam, the lamp must be made larger.

It is an object of the present invention to provide a light sourcecomprising an arc lamp which may have lower divergence and higherefficiency than have previously been possible, without the necessity ofincreasing the size of the lamp.

SUMMARY OF THE INVENTION

According to the present invention there is provided a light sourcecomprising an arc lamp including an anode and a cathode arranged toprovide an arc gap at the focal point of a conic reflector, and furtherreflective means arranged to reflect part of the light emitted by thearc gap in directions which do intersect the conic reflector back intothe arc gap.

Thus by a light source in accordance with the invention, peripherallight produced by the arc gap is directed back into the arc gap,reducing the divergence of the output beam, and increasing thetemperature and, hence the efficiency, of the arc.

The conic reflector is preferably a parabolic reflector.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of a light source in accordance with the invention willnow be described by way of example only with reference to theaccompanying drawings in which:

FIG. 1 is a schematic, partially sectioned side view of a prior artlight source as has already been described herebefore;

FIG. 2 is a schematic, partially sectioned side view of a light sourcein accordance with the invention;

FIG. 3 is a side view equivalent to the view of FIG. 2 showing a numberof light paths within the light source; and

FIG. 4 is an overview of a light source in accordance with an embodimentof the invention incorporated in a projection system.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 2, the light source in accordance with theinvention is an adaptation of the light source described in relation toFIG. 1. However, FIG. 2 is simplified in relation to FIG. 1 for the sakeof clarity.

In the light source shown in FIG. 2, an arc gap 201 is defined between acathode 203 and an anode 205, the arc gap being positioned at the focalpoint of a first parabolic concave reflector 207 which is truncated atits focal plane FP. The arc gap is contained in a xenon containingatmosphere enclosed in a housing partly defined by the parabolicreflector 207 and partly by a metallic block 209 whose inner surface ismachined to provide both a second spherical reflector 211 and a thirdconical reflector 213. The spherical reflector 211 and conical reflector213 are both oppositely directed to the parabolic reflector 207 and areboth concentric with the focus of the parabolic reflector 207. A furthercentral conical reflector 215, whose reflective surface is parallel tothat of the conical reflector 213, is also provided.

As in the prior art arrangement the lamp is sealed by means of asapphire window 217 formed as a focusing lens. The window 217 may carryon both its inner and outer faces infra-red reflective coatings 219, 221whose functions will be described in more detail hereafter.

The anode 205 is mounted in a heat sink 223 which is secured to themetallic block 209 by a ceramic spacer 225.

The light source also includes other features of the arrangement shownin FIG. 1 such as a magnetic focusing arrangement, and cooling fins.These features have, however, been omitted from FIG. 2 for the sake ofclarity.

Referring now also to FIG. 3, as the parabolic reflector 207 does notextend beyond its focal plane FP, when an arc is struck between theelectrodes 203, 205 any light emitted forward of the focal plane FP willbe reflected by the spherical reflector 211 back to the arc gap 201. Anexample of this is shown as ray a in FIG. 3. Furthermore, the reflectivecoatings 219, 221 carried by the window 217 are designed to reflect highenergy infra-red radiation from the arc back to the arc gap 201. Thisreflected light will be absorbed in the arc plasma which will act as ablack body. The reflected light from the reflector 211 and the coatings219, 221 will increase the plasma temperature considerably, and hencethe radiation efficiency of the arc as the majority of energy returnedto the plasma will be re-radiated and reflected by the parabolicreflector 207 through the window 217 and out of the lamp. The outermostbeam, indicated in FIG. 3 by beams b' and b" transmitted forwardly bythe parabolic reflector 207 will be reflected by the conical reflector213 on to the outer edge of the conical reflector 215 for forwardtransmission through the window 217. The beams shown as c' and c" inFIG. 3 represent the limit of the beam reflected by the conicalreflector 213 onto the innermost edge of the conical reflector 215.

Beams d' and d" in FIG. 3 represent beams which are reflected directlyby the parabolic reflector 207 out through the window 217, these beamsthus representing the outer limit of the output beam produced by thelamp. Light between d' and d", such as beams e' and e" in FIG. 3, willalso be reflected directly out of the window 217 by the parabolicreflector 207.

Thus, it can be seen that the combination of the long focal lengthparabolic reflector 207 and the spherical reflector 211 enable a greaterlight collection, leading to higher arc efficiency and lower beamdivergence than has previously been possible. It is found that in alight source in accordance with the invention, divergence of the outputbeam is roughly halved. As the maximum diameter of the output beam isreduced compared to prior art arrangements, the more compact beamenables a smaller window 217 to be used. In view of the use of sapphireas the window 217, and the necessary machining to form a lens, thisleads to a considerable cost reduction. It is also found that theoverall dimension of the light source can be reduced such that thelength of the lamp is around 70% of prior art light sources.

It is particularly convenient for the anode holder and parabolicreflector 207 to be both fabricated from a single block of metal, thisalso forming part of the housing for the light source. Likewise it isconvenient for the spherical surface 211 and conical surface 213 also tobe formed from the same piece of metal, this forming a further part ofthe housing. It will however be appreciated that the various componentsmay be formed separately and assembled together. With regard to theanode and cathode these will generally have tungsten surfaces at theirtips in order to withstand the high temperatures reached by the arc.This may be achieved by the use of conical tungsten tips 227, 229 asindicated in FIG. 2 or by tungsten inserts as 301, 303 as indicated inFIG. 3.

Turning now to FIG. 4, this figure illustrates the use of a light sourcein accordance with an embodiment of the invention in a projection systemincorporating three deformable mirror devices 401, 402, 403, each devicebeing effective to produce a spatially modulated light beam of adifferent primary colour. A light source 405 in accordance with theinvention is arranged to generate light along an incident light pathonto and through a pair of dichroic mirrors 407, 409. The first mirror407 is arranged to transmit red and green light, and reflect blue lightonto the first deformable mirror device 401. The second dichroic mirror409 is arranged to transmit green light onto the second deformablemirror device 403 and reflect red light onto a third deformable mirrordevice 405.

Dependent on address signals applied to the three deformable mirrordevices 401, 403, 405, each device 401, 403, 405 is effective to reflecta spatially modulated beam back along the optical axis of a projectionlens 411, the remaining light being reflected towards a beam dump notshown!.

The dichroic mirrors 407, 409 also cross the optical axis of theprojection lens so as to combine the spatially modulated blue, green andred light reflected from the deformable mirror devices 401, 403, 405.Thus a spatially modulated colour image is directed through theprojection lens 411 onto a screen 413, thereby producing a colourdisplay representative of the address signals supplied to the deformablemirror devices 401, 403, 405.

Thus it can be seen that a light source in accordance with the inventionhas particular applicability in such a projection system as the lightsource is capable of providing a substantially uniform low-divergencelight beam. Furthermore the light source is particularly efficient aslight which is reflected forward of the focal plane of the parabolicreflector which would normally not be intercepted is used rather thancausing overheating of the light source housing. Furthermore as thedivergence of the light source is reduced, a small diameter illuminationpatch is possible at the spatial light modulators, thus improving theefficiency of coupling of the light source output through the system tothe projection screen.

It will be appreciated however, that whilst a light source in accordancewith the invention has particular application in a projection systemincluding tiltable mirror devices, the light source is equallyapplicable to the illumination of other forms of spatial lightmodulators such as liquid crystals. Equally the light source inaccordance with the invention may find use on other forms of projectionsystems, or in other applications altogether.

It will be appreciated that it is particularly effective for the conicreflector 207 to be parabolic to produce the initial focusing of theoutput beam. However it is possible for the reflector to be ellipticalalthough this will complicate the optical design.

I claim:
 1. A light source comprising a sealed beam arc lamp including afirst conic reflector, and an anode and a cathode which between themdefine an arc gap, the arc gap being positioned around the focal pointof the first conic reflector such that light from the arc gap which isreflected from the first conic reflector is transmitted in asubstantially parallel beam towards an output window of the arc lamp,and a second spherical reflector concentric with the focal point of thefirst conic reflector having an aperture effective to permittransmission of said parallel beam towards the output window arrangedbetween the arc gap and the output window, the second spherical surfacebeing arranged to reflect light back into the arc gap, and third andfourth opposed conic reflector light towards the center of the outputbeam in order to compensate for light obscured by one of said cathodeand anode.
 2. A light source according to claim 1, in which the firstconic reflector is a parabolic reflector.
 3. A light source according toclaim 1, in which the first conic reflector does not extend past a focalplane of the conic reflector.
 4. A light source according to claim 1,including at least one infra-red radiation reflective layer carried onthe output window for the light source.
 5. A light source according toclaim 4, wherein inner and outer surfaces of said output window carrysaid reflective layers.
 6. A light source according to claim 1, whereinthe outer of said third and fourth opposed conic reflectors is integralwith said second spherical reflector.
 7. A light source according toclaim 6, wherein the second spherical reflector and said outer conicreflector form part of the housing for the light source.
 8. A projectionsystem including the light source according to claim
 1. 9. A lightsource according to claim 1, wherein the first conic reflector formspart of the housing for the light source.